1. Field of Invention
The present invention relates generally to data communication systems. More particularly, the present invention relates to systems and methods for enabling path-protected segments of a circuit to include line-protected links.
2. Description of the Related Art
The demand for data communication services is growing at an explosive rate. Much of the increased demand is due to the fact that more residential and business computer users are becoming connected to the Internet. Furthermore, the types of traffic being carried by the Internet are shifting from lower bandwidth applications towards high bandwidth applications which include voice traffic and video traffic.
To address the demand for data communication services, the use of optical networks, such as a synchronous optical network (SONET) is becoming more prevalent. One type of SONET network, or one type of network that is subject to SONET standards, is a time division multiple access (TDM) network. TDM networks generally allocate single channels or lines to be used amongst multiple users, or customers of data communication services. The single channels may each be divided into slots of time during which each user has access to the single channels. Many TDM networks may be configured to include a path protected mesh network (PPMN). A PPMN is a mesh of unprotected links with substantially no physical topological constraint. One example of a PPMN is a unidirectional path-switched ring (UPSR), which is effectively a PPMN with a ring-like topology. A UPSR or a virtual UPSR in a PPMN, as will be appreciated by those skilled in the art, provides for two different paths between a source and a destination. The source and the destination each select traffic from one of the paths based upon, for example, signal presence and signal quality.
Although a network such as a TDM network is generally designed to ensure that information may be transferred within the network reliably, there are times where network components may fail. Failures of network components may cause received signals to have a relatively high bit-error rate or, in some cases, cause a signal to be completely lost. In SONET, mechanical failures and equipment failures may occur. Mechanical failures include the effects of vibrations which affect optical connections, and bends in links or fibers which may result in degraded signal levels. Equipment failures may include, but are not limited to, failures of lasers which transmit optical signals, failures of links or channels over which signals are transmitted, and failures of nodes which are interconnected by links.
In order to compensate for link failures, links may be protected. A protected link is a link which has two fibers or cables over which signals may be transmitted substantially simultaneously. One example of a protected link is a link with “1+1” protection. Another example of a link with line protection is a bi-directional line switched ring (BLSR) link. A link with 1+1 protection, or a “1+1 link,” is made up of two unprotected links, i.e., sub-links. When an optical signal is to be transferred across a 1+1 link from a source node to a destination node, the optical signal is transferred across both sub-links of the 1+1 link. That is, the optical signal is transferred redundantly across both sub-links. The use of two sub-links within a 1+1 link enables a choice to be made regarding which optical signal received on the destination node is to be accepted by the destination node. Typically, the signal with the lower bit-error rate is accepted.
Some links between nodes are unprotected as it is generally less expensive to transfer signals across an unprotected link than it is to transfer signals across a protected link. While an unprotected link typically enables data to be transferred successfully, if a particular unprotected link between two nodes fails, then there is no alternate link over which data may be transferred between the two nodes.
In general, nodes may have many links therebetween. FIG. 1 is a diagrammatic representation of nodes in a TDM network that are connected by links which include links with 1+1 protection and unprotected links in a mixed protection domain. Within a network 100, nodes 102 are substantially interconnected with links 106. Links 106a, 106b are protected links which have 1+1 protection, while links 106c-i are unprotected links. As discussed above, protected links 106a, 106b may each include two unprotected sub-links. Since protected links 106a, 106b have 1+1 protection, if one of the sub-links in either of protected links 106a, 106b fails, a signal being carried on protected links 106a, 106b may still be transmitted over the sub-link associated with the failed sub-link.
When node 102a is an overall source node, and node 102d is an overall destination node, a customer may specify to a network administrator whether he prefers a protected path or an unprotected path between nodes 102a and 102d. When a customer specifies a protected path or a protected circuit between nodes 102a and 102d, a primary path 204 between nodes 102a and 102d may be defined to include 1+1 links 106a and 106b, and link 106c, as shown in FIG. 2. When primary path 204 is defined between nodes 102a, 102d, the segment which includes 1+1 links 106a and 106b, i.e., the segment between node 102a and node 102c, is substantially automatically protected. However, the segment between node 102c and node 102d is an unprotected segment as it includes unprotected link 106c. Hence, in order for an overall protected path to be defined between nodes 102a and 102d, a segment which is suitable for protecting the segment between nodes 102c and 102d that includes unprotected link 106c must be found.
A segment between nodes 102c and 102d which is suitable for protecting the segment that includes unprotected link 106c includes unprotected links 106d-h, and passes through nodes 102e-h. Therefore, primary path 204 is effectively protected by an alternate path 208 which begins at node 102a and includes links 106d-h. 
Although the shortest, lowest cost path from node 102a to node 102d would utilize single unprotected link 106i, such a path would not be usable, i.e., such a path may not be used as a part of a protected circuit. The cost associated with a path may generally depend upon at least one of factors such as the number of hops in a path, the distance traversed by the path, the length of fibers used in the path, and the administrative cost associated with the fibers in the path. The inability to use unprotected link 106i is due, at least in part, to the fact that 1+1 links 106a, 106b may not be used to protect unprotected link 106i. Since a UPSR may not utilize a segment which includes 1+1 links 106a, 106b to protect an unprotected path, the absolute shortest, lowest cost path between node 102a and node 102d may not be used to send traffic between node 102a and node 102d when a protected circuit is desired.
The inability to use the shortest available path to send traffic between a source node and a destination node often causes inefficiencies within a network, particularly when there is available bandwidth associated with the shortest available path. This is due to the fact that path protected segments of protected circuits may not contain line-protected links, and the fact that segments of an alternate path which protects a primary path also may not include segments which include line-protected links.
Therefore, what is needed is an efficient method and apparatus for allowing an option for line-protected links, e.g., links which have 1+1 protection, to be included in path-protected segments of a protected circuit. That is, what is desired is an efficient and accurate system which enables an end-to-end path between a source and a destination to be substantially optimized while enabling line-protected links to be included in path-protected segments of a protected circuit if an operator chooses to enable the option of using line-protected links.